This invention relates to a plastic lens having a stepped segment.
The lenses made of organic glass have come into wide use replacing the inorganic glass made lenses in recent years. Organic glass has many advantages over inorganic glass, such as light weight, high stability, good workability and dyeability, and as is well known in the art, diethylene glycol bisallyl carbonate is already widely utilized for the production of organic glass.
However, problems were involved in application of cast polymerization technique to the manufacture of lenses having a stepped segment; according to this technique, stress would concentrate at the stepped segment portion of the lens at the time of polymerization shrinkage, making the lens and mold susceptible to break and thus badly reducing the productivity.
In order to solve these problems, the following methods have been proposed and attempted: a method in which molding is carried out by controlling the degree of polymerization of diethylene glycol bisallyl carbonate and a method in which an excessively longer time is used for the polymerization than in the conventional methods to improve the moldability. However, the former method had the disadvantage that because of its extremely low surface hardness, the product might be splitted at the time of annealing or the product (lens) might prove unable to undergo surface coating, while the latter method had the problems of very poor productivity and liability of the lens to get tinted in yellow.